readme: strange phrase re tarantool 2.4.0

> # Limitations
> <...>
> 2. This module does not support Tarantool 1.6.x starting with 2.4.0.
>    Since then it uses Tarantool 1.7 protocol features.

It was edited the last time 2017-12-18, when tarantool-2.4 did not exist (2.0.4 was released 2018-02-15).